NetNews Offline 2/NetNews Offline Volume 2.iso/news/comp/lang/c-part1/5797

Internet Message Format | 1996-08-05 | 2.2 KB

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From: Lawrence Kirby <[e-mail]>
Newsgroups: comp.lang.c
Subject: Re: printing pid_t...
Date: Tue, 20 Feb 96 23:34:04 GMT
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In article <4gcv6f$[e-mail]>
           [e-mail] "Godfrey Degamo" writes:

>Hello.  Sorry to bother you with this question.  I don't know
>if this belongs in comp.unix.programmers or here.
>
>Anyways, I want to read and write pid_t to a text file.  The problem
>is how is pid_t type defined?  As an int, long, unsigned int or
>unsigned long.  (I think that's all the types that pid_t can be...)
>Can I be safe to assume unsigned long and use %lu for printing and
>scanning?

A major reason for creating a type alias such as pid_t is to allow the
implementation to choose the most appropriate type for it. While pid_t
itself is Unix/POSIX specific the problem of printing values such as this
is a more general one. For example the C language defines a type size_t
which is the type of the result of the sizeof operator. size_t is defined
by the standard to be an 'unsigned integral type' but you don't know which.
However you can cast any unsigned integral type to unsigned long while
preserving the value so the portable way of printing the size of a variable
x is:

   printf("%lu\n", (unsigned long) sizeof x);

Similarly the C language standard defines ptrdiff_t to be "the signed
integral type of the result of subtracting two pointers" so you could
portably write:

   printf("%ld\n", (long)(ptr2-ptr1));

I don't know if pid_t is defined to be a signed type or not but its usage
in fork() and especially kill() suggests that it won't in practice hold
a value which can't be represented by a long, so I'd use a form equivalent to
the 2nd example. Direct further discussion of pid_t to comp.unix.programmer.

--
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Lawrence Kirby | [e-mail]
Wilts, England | [e-mail]
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